[dev.ssa] cmd/compile/internal/ssa: more checks on ssa structure

Make sure all referenced Blocks and Values are really there.
Fix deadcode to generate SSA graphs that pass this new test.

Change-Id: Ib002ce20e33490eb8c919bd189d209f769d61517
Reviewed-on: https://go-review.googlesource.com/13147
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>

2 files changed, 40 insertions, 5 deletions

diff --git a/src/cmd/compile/internal/ssa/check.go b/src/cmd/compile/internal/ssa/check.go
index 1f6ffc0129..668828fcd1 100644
--- a/src/cmd/compile/internal/ssa/check.go
+++ b/src/cmd/compile/internal/ssa/check.go
@@ -137,13 +137,36 @@ func checkFunc(f *Func) {
 		}
 	}
 
+	// Check to make sure all Blocks referenced are in the function.
+	if !blockMark[f.Entry.ID] {
+		f.Fatalf("entry block %v is missing", f.Entry)
+	}
 	for _, b := range f.Blocks {
-		if b.Control != nil {
-			if !valueMark[b.Control.ID] {
-				f.Fatalf("control value for %s is missing: %v", b, b.Control)
+		for _, c := range b.Preds {
+			if !blockMark[c.ID] {
+				f.Fatalf("predecessor block %v for %v is missing", c, b)
+			}
+		}
+		for _, c := range b.Succs {
+			if !blockMark[c.ID] {
+				f.Fatalf("successor block %v for %v is missing", c, b)
 			}
 		}
 	}
+
+	// Check to make sure all Values referenced are in the function.
+	for _, b := range f.Blocks {
+		for _, v := range b.Values {
+			for i, a := range v.Args {
+				if !valueMark[a.ID] {
+					f.Fatalf("%v, arg %d of %v, is missing", a, i, v)
+				}
+			}
+		}
+		if b.Control != nil && !valueMark[b.Control.ID] {
+			f.Fatalf("control value for %s is missing: %v", b, b.Control)
+		}
+	}
 	for _, id := range f.bid.free {
 		if blockMark[id] {
 			f.Fatalf("used block b%d in free list", id)
diff --git a/src/cmd/compile/internal/ssa/deadcode.go b/src/cmd/compile/internal/ssa/deadcode.go
index 04e5b71ceb..426e6865c0 100644
--- a/src/cmd/compile/internal/ssa/deadcode.go
+++ b/src/cmd/compile/internal/ssa/deadcode.go
@@ -6,7 +6,6 @@ package ssa
 
 // deadcode removes dead code from f.
 func deadcode(f *Func) {
-
 	// Find all reachable basic blocks.
 	reachable := make([]bool, f.NumBlocks())
 	reachable[f.Entry.ID] = true
@@ -85,6 +84,11 @@ func deadcode(f *Func) {
 			if len(b.Values) > 0 {
 				b.Fatalf("live values in unreachable block %v: %v", b, b.Values)
 			}
+			s := b.Succs
+			b.Succs = nil
+			for _, c := range s {
+				f.removePredecessor(b, c)
+			}
 			f.bid.put(b.ID)
 		}
 	}
@@ -108,14 +112,22 @@ func (f *Func) removePredecessor(b, c *Block) {
 		b, c := work[0][0], work[0][1]
 		work = work[1:]
 
-		// find index of b in c's predecessor list
+		// Find index of b in c's predecessor list
+		// TODO: This could conceivably cause O(n^2) work.  Imagine a very
+		// wide phi in (for example) the return block.  If we determine that
+		// lots of panics won't happen, we remove each edge at a cost of O(n) each.
 		var i int
+		found := false
 		for j, p := range c.Preds {
 			if p == b {
 				i = j
+				found = true
 				break
 			}
 		}
+		if !found {
+			f.Fatalf("can't find predecessor %v of %v\n", b, c)
+		}
 
 		n := len(c.Preds) - 1
 		c.Preds[i] = c.Preds[n]